Load control and holding valve

ABSTRACT

A load control and holding valve adapted to be installed between a directional control valve and a fluid motor to control flow of fluid therethrough to and from the motor, said valve being characterized in that a main valve member therein functions in the manner of a check valve member to permit flow of fluid under pressure from the directional control valve to the fluid motor as when a load thereon is being raised and functions in the manner of a pilot operated relief valve member to support a load on the fluid motor against downward drifting as when the directional control valve is in neutral &#39;&#39;&#39;&#39;off&#39;&#39;&#39;&#39; position except in the case of load inertia pressure, shock pressure, or thermal expansion pressure of magnitude sufficient to open a pilot relief valve member in said valve thus to effect opening of the main valve to relieve such excess pressure. The valve herein is further characterized in that when it is desired to open the main valve member to permit return flow therethrough from the fluid motor to the directional control valve a piston in the valve exposed to pilot pressure in the pressurized side of the motor is effective to open the pilot relief valve member thus to permit opening of the main valve member for such return flow. In the event of reduction of pressure in the pressurized side of the motor as by a negative load thereon, the decreased pressure acting on the piston will permit reclosing or throttling of the pilot valve member with consequent closing or throttling of the main valve member, thus to prevent cavitation of the motor.

1111 3,795,255 Mar. 5, 1974 gh to and v LOAD CONTROL AND HOLDING VALVE I [75] Inventors: Thomas J. Malott, Mentor; Hugh J.

motor to control flow of fluid therethrou from the motor, said valve being characteri zed in that a main valve member therein functions in the manner Stacey, Willoughby, both of Ohio of a check valve member to permit flow of fluid under pressure from the directional control valve to the fluid I73] Assignee: Parker-Hannifin Corporation,

Cleveland, Ohio motor as when a load thereon is being raised and funcgainst ownward drifting as when the directional control valve is in neutral off load inertia pressure tions in the manner of a pilot operated relief valve member to support a load on the fluid motor a d position except in the case of 1 2 7 9 l w um J2 o N .ml P mP FA U i 22 52 us. 137/102, 91/420, 137/5962 l Pressure/,0 thermal 51 int. Fl5b 13/042 Pressure of magmt-"de sufficem Open a [58] Field of Search" 137/87 102 106 596 5962. pilot relief valve member in said valve thus to effect opening of the main valve to relieve such excess'pressure. The valve herein IS further characterized in that when it is desired to open the main valve member to [56] References Cited 7 UNITED STATES PATENTS permit return flow therethrough from the fluid motor to the directional control valve a piston in the valve Lee et exposed to pilot pressure in the pressurized side of the motor is effective to open the pilot relief valve member thus to permit opening of the main valve member for such return flow. [n the event of reduction of pres- Finlayson..... Stewart et al. Olen sure in the pressurized side of the motor as by a negative load thereon, the decreased pressure acting on the piston will permit reclosing or throttling of the pilot xx x 0 1170 2 2 4U l4 U9 9 a m9 Primary Examiner-Robert G. Nilson Attorney, Otto valve member with consequent closing or throttling of the main valve member, thus to prevent cavitation of the motor.

Maky, Renner &

Agent, or FirmDonnelly,

57 ABSTRACT A load control and holding valve ada pted to be installed between a directional control valve and a fluid 12 Claims, 2 Drawing Figures I LOAD CONTROL AND HOLDING VALVE BACKGROUND OF ,THE INVENTION In known load control and holding valves such as disclosed for example in the -Martin U. S. Pat. No. 3,595,264, granted July 27, 1971, there is provided a unitary counterbalance valve member with a separate check valve member therewithin, both valve members being closed to lock a fluid motor against downward drift of a load acting thereon. The counterbalance valve comprises a body having a stepped bore with a seat at its small end and a stepped counterbalancedvalve member which is springbiased into engagement with the body seat and which has a first relatively small annular area exposed to motor load pressure to open said valve member in the event of overload due to shock load, load inertia, or thermal expansion of the fluid between the motor and the counterbalance valve. The counterbalance valve member also has a second relatively larger annular area which is exposed to pilot pressure derived from the pressure side of the motor to open the return side'of the motor when the load is raised and to throttle or shut off the return side when the load tends to run ahead of thepump. The counterbalance valve member has the check valve in its small end which permits flow to the motor when it is desired to reverse the direction of actuation of the motor.

As apparent, this construction with the check valve seat disposed coaxially within the counterbalance valve member seat requires a relatively heavy counterbalance valve member biasing spring to perform its shock, inertia, and thermal relief functions.

SUMMARY or THE INVENTION In contradistinction to.the foregoing, the present invention entails the use of pilot operated relief. valve principles which enable the combination of the known counterbalance valve member and the separate check valve member into a single differential pressure operated main valve member thus also permitting the use of relatively weak springs, one of which biases the main valve member to seated position with the assistance of load pressure to perform the load holding function and yields to permit opening of the. main valve member as I a check valve member whenthe load is to be raised,

and the other of which biases a relatively small area pilotrelief valve member into seated position against motor, load pressure and yields by excess pressure thereon or by force exerted thereon by a piston exposed to pilot pressure from the pressurized side of the motor for opening of the pilot relief valve member and consequent opening of the main valve member to permit flow from the return side of the motor to the directional control valve.

It is another object of this inventionto provide a valve of the character indicated in which the actuating piston for the pilot reliefvalve member has one-way en-.

gagement with the pilot relicfvalve member so that when the piston is actuated by fluid pressure in the pressurized side of the fluid motor, it moves the pilot relief valve member away from its seat to permit opening of the main valve member for return flow from the fluid motor, and so that when there is a reduction of pressure in the pressurized side of the motor due to, for

example, a load on the motor tending to move faster than the pump can supply fluid to the pressurized side of the motor, the piston is moved by fluid pressure on the other side thereof to a stop position disengaged from the pilot relief valve member whereat the pilot relief valve member may hover to and away from its seat to permit movement of the main valve member toward its seat to throttle or discontinue return flow from the motor to the directional control valve thereby to decrease the speed of actuation of the motor.

Other objects and advantages of the present invention will appear hereinafter.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-section view of the load control and holding valve according to the present invention showing the same as employed in a hydraulic system comprising a fluid reservoir, a pump, a directional control valve and a double acting fluid motor, the load control and holding valve being operatively interposed between the directional control valve and the fluid motor; and

FIG. 2 is a schematic diagram showing two valves according to the present invention as embodied in a hydraulic system wherein the fluid motor load tends to run away from the pressure side of the motor in both directions of actuation of the double acting fluid motor.

DETAILED DESCRIPTION OF THE DRAWING In the hydraulic system illustrated in FIG. 1, there is provided a fluid reservoir 1, a pump 2, a four-way directional'control valve 3, a load control and holding valve 4, and a fluid motor 5. The directional control valve 3 is of well known form except that in the present case it is preferred that it have a restricted bypassfi in neutral position as shown in FIg. l. The directional control valve 3' has two operating positions in one of which fluid under pressure delivered by the pump 2 is conducted to the rod end of the fluid motor 5 via valve 3 and conduit 7 and return fluid from the head end of the fluid motor 5 is conducted to the reservoir 1 via conduit 8, loadcontrol and holding valve 4, conduit 9, and valve 3, and in the otherofwhich fluid under pressuredelivered by the pump is conducted through conduit 9, load control and holdingvvalve 4, and conduit 8, into the head end of the motor 5 and return fluid from the rod end of the motor 5 is conducted to the reservoir 1 via conduit 7 and valve 3. i

.The load controland holding valve 4 comprises a housing 10 formed with the three ports viz,'a pilot pressure port 11 connected to conduit 7, a motor port 12 connected to conduit 8, and a valve port 14 connected to conduit 9. The housing, in addition, has a stepped bore 15 which isintersected axially therealong by passages 16 to 19 which respectively communicate with the ports 11, 14, 12, and 14.

Within the stepped bore 15 is a pilot operated relief valve assembly 20 which comprises a main valve seat 21 against which a main valve member 23' is biased by the spring 24, a tubular body 25 which is screwed into the housing 10 and which has a'pilot relief valve seat 26 against which the pilot valve member 27 is biased by the spring 28. The spring guide and follower 29 is axially'adjustable in said body 25 by means of theadjusting screw 30.

A tubular piston 31 is disposed within said body 25 and around said spring follower 29 and has an inturned flange 32 at its lower end which radially overlaps the flange 34 of the pilot relief valve member 27, said piston'31 having an annular area A which is exposed to pilot pressure in the pilot port 11 and passage 16 derived from conduit 7. The piston 31 in its lowermost position is engaged with a shoulder 35 in body 25 so as to be clear of pilot relief valve member 27 when the latter is engaged with seat 26.

The passage 17 communicates with the downstream side of the pilot valve seat 26 so that when the conduit 8 is the return conduit, the opening of the pilot relief valve member 27 will vent the chamber 36 behind the main valve member 23 faster than fluid can enter through the orifice 37 in said main valve member 23, whereby pressure in the port 12 and passage 18 will move the main valve member 23 upwardly against its biasing spring 24 thus to open communication through the seat 21 so that return fluid can pass from the head end of the motor through the valves 4 and 3 to the reservoir 1. When the conduit 9 is the pressure supply conduit, pressure in the passage 17 communicates with the downstream side of the pilot relief valve member 27 and the chamber 38 behind the piston 31 (via openings in the piston 31 and follower 29) to hold said member 27 and piston 31 in the positions shownfWhen the conduit 9 is thus pressurized, the main valve member 23 will operate as an ordinary check valve member to permit flow of fluid through the seat 21, passage 18, motor port 12, and conduit 8 into the head end of the motor 5 to raise the load thereon.

When the directional control valve 3 is shifted to the right position, fluid under pressure will be delivered through the conduit 7 to the rod end of the motor 5 and also through the pilot port 11 and passage 16 to act on the annular area A of the piston 31 and when'the pilot pressure reaches a value to cause the piston 31 to move upwardly to engage the pilot relief valve member 27 and to lift the latter from its seat- 26 overcoming the bias of the spring 28, the chamber 36 behind the main valve member 23 will be ventedthrough the passage 17 to the reservoir 1 via the valve port 14, conduit 9 and the directional control valve 3 thereby to lower the pressure in the chamber 36 to avalue'such that the pressure drop across the orifice 37 will effect upward movement of the main valve member 23 overcoming the bias of the spring 24, thusto permit return flow of fluid from the head end of the motor to the reservoir 1 via conduit 8, motor port 12, passages 18 and 19, valve port 14, conduit 9, and valve 3.

When the valve 3 is in neutral position as shown and valve 4 is in the load holding position as shown in FIG. 1, the main valve member 23 is engaged with seat 21 and thus downward drift of a load on motor 5 is prevented. In the event of thermal expansion and pressure buildup of the fluid trapped between the motor 5 and the valve 4, the high pressure in the chamber 36 will urge the pilot relief valve member 27 upwardly away from its seat 26 thus to vent the chamber 36 faster than fluid can be replenished through the orifice 37, whereby the main valve member 23 is moved up from its seat 21 to relieve such excess pressure.

When the directional control valve 3 is shifted to the right from neutral position, the conduit 7 is pressurized to conduct fluid under pressure into the rod end of the motor 5 and in the event ofa heavy load on said motor 5 the displacement of the rod end may tend to exceed the flow capacity of pump 2 with an accompanying decrease in fluid pressure .in the conduit 7 and in the pilot port 11 and passage 16 with the result that when the pressure decreases to a value such that the spring 28 may urge the pilot relief valve member 27 downwardly into engagement with its seat 26 the pressures on opposite sides of the main valve member 23 become equalized through the orifice 37 whereby the force exerted by the biasing spring 24 with the greater area of the chamber 36 side of said member 23 urges the main valve member 23 to closed position or toward closed positionthus to prevent cavitation of the rod end of said motor 5. To minimize hunting or fluttering of the piston 31 and pilot relief valve member 27, the pilot passage 16 has an orifice 16' to prevent rapid changes in pilot pressure acting on the area A of the piston 31. While the main valve member is open the shifting of the valve 3 from the right position back to neutral position builds up a back'pressure through bypass orifice 6 to decelerate the downward actuation of the load on motor 5. The increased pressure in valve port 14 is conducted through passage 17 to communicate with chamber 38 and to act on pilot relief valve member 27 to effect closing of the main relief valve member 23.

Referring now to FIG. 2, there is shown a fluid motor 40 which is subjected to a negative load on both sides of the center position. To incorporate this invention in such a system, two valves 4 are interconnected as shown so that pressurized fluid flows through one valve 4 with pilot pressure being conducted to the pilot port 11 and passage 16 of the other valve 4 to enable return flow from the motor 40 through the last-mentioned valve 4 and to a'reservoir 1 via a directional control valve 3 such as that shown in FIG. 1. Thus in the FIG. 2 installation, cavitation of both ends of the fluid motor 40 is prevented by blocking or throttling the return flow of the motor 40 to decrease its speed so as to prevent cavitation in the pressurized side thereof.

We, therefore, particularly point out and distinctly claim as our invention: I

l. A valve assembly comprising a body having motor, valve, and pilot ports, and a main passage connecting said valve and motor ports; a normally closed main valve controlling flow of fluid through said main passage having areas on one said thereof exposed to fluid pressure in said motor and valve ports respectively, said main valve and body defining a pressure chamber havingrestricted communication with said motor port and less restricted communication with said valve port via a control passage connecting said chamber and valve port; said main valve having an area on the other side thereof exposed to fluid pressure in said chamber;-a normally closed pilot valve controlling flow of fluid through said control passage; and means operated by fluid pressure in said pilot port having a surface radially overlapped with said pilot valve for pulling open said pilot valve to thus decrease the fluid pressure in said chamber relative to the fluid pressure in said motor port whereby predominant fluid pressure in said motor port opens said main valve for flow of fluid through said main passage from said motor port to said valve port. 2. A valve assembly comprising a body having motor, valve, and pilot ports, and a main passage connecting said valve and motor ports; a normally closed main valve controlling flow of fluid through said main passage having areas on one side thereof exposed to fluid pressure in said motor and a valve ports respectively, said main valve and body defining a pressure chamber having restricted communication with said motor port and less restricted communication with said valve port via, a control passage connecting said chamber and valve port; said main valve having an area on the other side thereof exposed to fluid pressure in said chamber; a normally closed pilot valve controlling flow of fluid through said control passage; and means operated by fluid pressure in said pilot port for opening saidpilot valve to thus decrease the fluid pressure in said chamber relative to the fluid pressure in said motor port whereby predominant fluid pressure in said motor port opens said main valve ,for flow of fluid through said main passage from said motor port to said valve port; said pilot valve being spring biased to closed position and being operable to open position independently of said last-named means by. apredetermined pressure in said chamber, I

3. The valve assembly of claim 2 wherein decrease of said fluid pressure in said pilot port to a predetermined value permits spring biased closing of said pilot valve thus to effect closing of said main valve member to block flow of fluid through said main passage from said motor port to said valve port; v I

4. A valve assembly comprising a body having motor, valve, and pilot ports, and a main passage connecting said 'valve and motor ports; a normallyclosed main valve controlling flow of fluid through said main passage having areas on one side thereof exposed to fluid pressure in said motor and valve ports respectively, said main valve and body defining a pressure chamber having restricted communication with said motor port and less restricted communication with saidv valve port via a control passage connecting said chamber and valve port; said main valve having anarea on the other side thereof'exposed to fluid pressure in said chamber; a normally closed pilot valve' controlling flow of fluid through said control passage; and means operated by fluid pressure in said pilot port for opening said pilot valve to thus decrease the fluid pressure in said chamber relative to the fluid pressurein said motor port whereby predominant fluid pressure in said motor port opens said main valve for flow of fluid through said main passage from said motor port to said valve port; said pilot valve constituting a pilot relief valve which opens said control passage independently of said fluid pressure operated means to open said main ,valve 'to'relieve excess fluid'pressure in said motor port to said valve port. V

5. A load control and holding valve comprising a housing having motor, valve,v and pilot ports, a main passage having a main valve seat communicating said valve and motor ports with each other, and a control passage having a pilot valve seat communicatinga chamber in said housing with said valve port; a main valve membenmovable in said housing into engagement withsaid main valve seat to close said main passage and out of engagement with said main valve seat to permit flow of fluid from said valve port to said motor port and vice versa, said main valve member defining with said housing said chamber which has restricted communication with-said motor port; a pilot valve member movable .in said housing into engage. ment with said pilot valve seat to close said control passage to equalize the fluid pressures in said chamber and said motor port through such restricted communication and out of engagement with said pilot valve seat to communicate said chamber with said valve port via said control passage thereby to decrease the fluid pressure in said chamber relative to fluid pressure in said motor port for opening of said main valve member by predominant pressure in said motor'port for flow of fluid through said main passage from said motor port to said valve port; and fluid pressure operated means in said housing operative in response to fluid pressure in said pilot port to pull said pilot valve member out of engagement with said pilot valve seat.

6. A load control and holding valve comprising a housing having motor, valve, and pilot ports, a main passage having a main valve seat communicating said valve and motor ports with each other, and a control passage having a pilot valve seat communicating a chamber in said housing with said valve port; a main valve member movable in said housing into engagement with said main valve seat to close said main passage and out of engagement with said main valve seat to permit flow of fluid from said valve port to said motor port and vice versa, said main valve member defining with said housing said chamber which has restricted communication with said motor port; a pilot valve. member movable in said housing into engage;

'ment with said pilot valve seat to close said control passage to equalize the fluid pressures in said chamber and said motor port through such restricted communication and out of engagement with said pilot valve seat to communicate said chamber with said valve port via said control'passage thereby to decrease the fluid pressure in said chamber relative to fluid pressure in said motor port for opening of said main valve member by predominantpressure in said motor port for flow of fluid through said main passage from said motor port to said valve port; and fluid pressure operated means in said housing operative in response to fluid pressure in said pilot port to move said. pilot valve member out of engagement with said pilot valve seat; spring means bias- I ing said pilot valve member into engagement withsaid pilot valve seat whereby equal fluid pressures in said motor port and chamber retains said mainvalve member in engagement with said main valve seat thus to hold a load on a fluid motor adapted to be connected to said motor port until fluid pressure in said pilot port acting on said fluid pressure operated means is sufficient to overcome such' spring bias to move said pilot valve member out of engagementwith said pilot valve seat; said pilot valve member being movable out of engagement with said pilot valve seat independently of said fluid pressure operated means when the fluid pressure in said motor port and chamber exceeds a predetermined maximum which overcomes said spring bias thus to effect movement of said pilot valve member out of engagement with said pilot valve seat and consequent movement of said main valve member out of en gagement with said main valve seat to relieve excess fluid pressure in said motor port to said valve port via said main passage.

7. in a load control, and holding valve of the type wherein a valve housing has valve, motor, and pilot ports therein, and has valve means and fluid pressure operated valve actuating means therein which valve means opens communication. between said valve and motor ports responsive to predominant fluid pressure in said valve port for flow of fluid to said motor port and which opens communication between said valve and motor ports responsive to predetermined fluid pressure in said pilot port which acts on said valve actuating means for flow of fluid to said valve port, the improvement which comprises valve means and actuating means therefor including a unitary valve member which defines with said housing a pressure seating chamber having restricted communication with said motor port; and a pilot valve member which is actuated by said actuating means to decrease the fluid pressure in said chamber relative to fluid pressure in said motor port whereby predominant motor port fluid pressure in relation to chamber pressure and valve port pressure moves said unitary valve member to open communication between said motor and valve ports for flow of fluid from said motor port to said valve port, said unitary valve member further being operative to open communication between said valve and motor ports for flow of fluid from said valve port to said motor port when the fluid pressure in said valve port exceeds that in said motor port and said chamber when the pressure in said pilot port is less than such predetermined value; said housing having a control passage between said chamber and said valve port which is opened or closed by said pilot valve member according to whether fluid pressure in said pilot port is equal to or less than such predetermined value; said pilot valve member being spring biased to a position closing said control passage; and said pilot valve member being operative independently ofsaid actuating means to open said control passage responsive to fluid pressure in said motor port and chamber exceeding a predetermined maximum value overcoming such spring bias thus to relieve excess pressure for flow from said motor port to said valve port by consequent movement ofrsaid main valve member to open communication between said motor and valve ports.

8. A load control and holding valve comprising a housing having a first passage with a main valve seat; a pilot operated relief valve in said housing comprising a spring biased main valve member movable into and out of engagement with said main valve seat and defining with said housing the movable wall of a chamber having one side thereof in restricted communication with the other side thereof; said housing having a second passage communicating said chamber with said first passage on one side of said main valve seat and having a pilot valve seat therein; a spring-biased pilot valvemember movable into and'out of engagement said said pilot valve seat to open and close said second passage whereby when fluid under pressure on said other side of said main valve member and in said chamber attains a magnitude sufficient to move said pilot valve member out ofengagement with said pilot valve seat said chamber is vented through said second passage more rapidly than fluid can be replenished into said chamber from the other side of said main valve member whereby said main valve member is moved out of engagement with said main valve seat to relieve such pressure in said first passage on the other side of said main valve seat for flow in one direction through said first passage; said main valve member being moved a check valve member when the pressure of said one side of said main valve seat in said first passage exceeds the pressure on the other sideof said main valve seat thus to permit flow of fluid through said first passage in the opposite direction; said housing having a pilot pressure passage; a piston movable in said housing to engage said pilot valve member to move it out of engagement with said pilot valve seat when the pilot pressure in said pilot passage acting on said piston rises to a value sufficient to overcome the spring bias on said pilot valve member thus to again permit movement of said main valve member out of engagement with its seat for flow of fluid in such one direction through said first passage.

9. The valve of claim 8 wherein said piston and pilot valve member have radially overlapped surfaces which engage each other when said piston is moved responsive to such pilot pressure in said pilot passage.

10. The valve of claim 8 wherein decrease of such pilot pressure to a predetermined value permits spring biased movement of said pilot valve member into engagement with said pilot valve seat thus to effect movement of said main valve member into engagement with its seat to stop flow of fluid in said first passage in said one direction.

11. The valve of claim 8 wherein said first passage opens in a valve port adapted for connection with a four-way directional control valve and in a motor port adapted for connection with one motor port of a double acting fluid motor for flow of fluid into and from such motor via said first passage and said main valve seat; and wherein said pilot pressure passage iscommunicated with the other motor port which is connected to such directional control valve whereby when such other motor port of said motor is pressurized by operation of the directional control valve, pressure in said pilot pressure passage actuates said piston to open said pilot relief valve member as aforesaid.

12. The valve of claim 8 wherein said housing has a stepped bore in which said pilot operated relief valve is disposed with the first passage intersecting the smallest and second smallest bore portions of said bore, with said second passage intersecting the third smallest portion of said bore, and with said pilot passage intersecting the fourth smallest portion of said bore, said main valve seat being disposed between said smallest and second smallest bore portions, said main valve member being movable in sliding contact with the second smallest bore portion, and said pilotoperated relief valve comprising a tubular body secured in said housing with a first portion extending into said second smallest bore portion and sealed thereto, with a second portion sealingly engaged with the third smallest bore portion and a third portion having screw threaded engagement with the fourth smallest bore portion, said second passage intersecting the third smallest bore portion downstream of the pilot valve seat in said body, said pilot passage intersecting saidfourth smallest bore portion and acting on an annular area of said piston reciprocable in said body, said body having an axially adjustable member therein telescoped within said piston to adjust the spring biasing force against said pilot valve member, said adjustmentmembcr having a passage therethrough and said piston having a passage to communicate a chamber on the other side of said piston with said second passage to permit piston movement without trap- 

1. A valve assembly comprising a body having motor, valve, and pilot ports, and a main passage connecting said valve and motor ports; a normally closed main valve controlling flow of fluid through said main passage having areas on one said thereof exposed to fluid pressure in said motor and valve ports respectively, said main valve and body defining a pressure chamber having restricted communication with said motor port and less restricted communication with said valve port via a control passage connecting said chamber and valve port; said main valve having an area on the other side thereof exposed to fluid pressure in said chamber; a normally closed pilot valve controlling flow of fluid through said control passage; and means operated by fluid pressure in said pilot port having a surface radially overlapped with said pilot valve for pulling open said pilot valve to thus decrease the fluid pressure in said chamber relative to the fluid pressure in said motor port whereby predominant fluid pressure in said motor port opens said main valve for flow of fluid through said main passage from said motor port to said valve port.
 2. A valve assembly comprising a body having motor, valve, and pilot ports, and a main passage connecting said valve and motor ports; a normally closed main valve controlling flow of fluid through said main passage having areas on one side thereof exposed to fluid pressure in said motor and a valve ports respectively, said main valve and body defining a pressure chamber having restricted communication with said motor port and less restricted communication with said valve port via a control passage connecting said chamber and valve port; said main valve having an area on the oTher side thereof exposed to fluid pressure in said chamber; a normally closed pilot valve controlling flow of fluid through said control passage; and means operated by fluid pressure in said pilot port for opening said pilot valve to thus decrease the fluid pressure in said chamber relative to the fluid pressure in said motor port whereby predominant fluid pressure in said motor port opens said main valve for flow of fluid through said main passage from said motor port to said valve port; said pilot valve being spring biased to closed position and being operable to open position independently of said last-named means by a predetermined pressure in said chamber.
 3. The valve assembly of claim 2 wherein decrease of said fluid pressure in said pilot port to a predetermined value permits spring biased closing of said pilot valve thus to effect closing of said main valve member to block flow of fluid through said main passage from said motor port to said valve port.
 4. A valve assembly comprising a body having motor, valve, and pilot ports, and a main passage connecting said valve and motor ports; a normally closed main valve controlling flow of fluid through said main passage having areas on one side thereof exposed to fluid pressure in said motor and valve ports respectively, said main valve and body defining a pressure chamber having restricted communication with said motor port and less restricted communication with said valve port via a control passage connecting said chamber and valve port; said main valve having an area on the other side thereof exposed to fluid pressure in said chamber; a normally closed pilot valve controlling flow of fluid through said control passage; and means operated by fluid pressure in said pilot port for opening said pilot valve to thus decrease the fluid pressure in said chamber relative to the fluid pressure in said motor port whereby predominant fluid pressure in said motor port opens said main valve for flow of fluid through said main passage from said motor port to said valve port; said pilot valve constituting a pilot relief valve which opens said control passage independently of said fluid pressure operated means to open said main valve to relieve excess fluid pressure in said motor port to said valve port.
 5. A load control and holding valve comprising a housing having motor, valve, and pilot ports, a main passage having a main valve seat communicating said valve and motor ports with each other, and a control passage having a pilot valve seat communicating a chamber in said housing with said valve port; a main valve member movable in said housing into engagement with said main valve seat to close said main passage and out of engagement with said main valve seat to permit flow of fluid from said valve port to said motor port and vice versa, said main valve member defining with said housing said chamber which has restricted communication with said motor port; a pilot valve member movable in said housing into engagement with said pilot valve seat to close said control passage to equalize the fluid pressures in said chamber and said motor port through such restricted communication and out of engagement with said pilot valve seat to communicate said chamber with said valve port via said control passage thereby to decrease the fluid pressure in said chamber relative to fluid pressure in said motor port for opening of said main valve member by predominant pressure in said motor port for flow of fluid through said main passage from said motor port to said valve port; and fluid pressure operated means in said housing operative in response to fluid pressure in said pilot port to pull said pilot valve member out of engagement with said pilot valve seat.
 6. A load control and holding valve comprising a housing having motor, valve, and pilot ports, a main passage having a main valve seat communicating said valve and motor ports with each other, and a control passage having a pilot valve seat communicating a chamber in said housing with saId valve port; a main valve member movable in said housing into engagement with said main valve seat to close said main passage and out of engagement with said main valve seat to permit flow of fluid from said valve port to said motor port and vice versa, said main valve member defining with said housing said chamber which has restricted communication with said motor port; a pilot valve member movable in said housing into engagement with said pilot valve seat to close said control passage to equalize the fluid pressures in said chamber and said motor port through such restricted communication and out of engagement with said pilot valve seat to communicate said chamber with said valve port via said control passage thereby to decrease the fluid pressure in said chamber relative to fluid pressure in said motor port for opening of said main valve member by predominant pressure in said motor port for flow of fluid through said main passage from said motor port to said valve port; and fluid pressure operated means in said housing operative in response to fluid pressure in said pilot port to move said pilot valve member out of engagement with said pilot valve seat; spring means biasing said pilot valve member into engagement with said pilot valve seat whereby equal fluid pressures in said motor port and chamber retains said main valve member in engagement with said main valve seat thus to hold a load on a fluid motor adapted to be connected to said motor port until fluid pressure in said pilot port acting on said fluid pressure operated means is sufficient to overcome such spring bias to move said pilot valve member out of engagement with said pilot valve seat; said pilot valve member being movable out of engagement with said pilot valve seat independently of said fluid pressure operated means when the fluid pressure in said motor port and chamber exceeds a predetermined maximum which overcomes said spring bias thus to effect movement of said pilot valve member out of engagement with said pilot valve seat and consequent movement of said main valve member out of engagement with said main valve seat to relieve excess fluid pressure in said motor port to said valve port via said main passage.
 7. In a load control and holding valve of the type wherein a valve housing has valve, motor, and pilot ports therein, and has valve means and fluid pressure operated valve actuating means therein which valve means opens communication between said valve and motor ports responsive to predominant fluid pressure in said valve port for flow of fluid to said motor port and which opens communication between said valve and motor ports responsive to predetermined fluid pressure in said pilot port which acts on said valve actuating means for flow of fluid to said valve port, the improvement which comprises valve means and actuating means therefor including a unitary valve member which defines with said housing a pressure seating chamber having restricted communication with said motor port; and a pilot valve member which is actuated by said actuating means to decrease the fluid pressure in said chamber relative to fluid pressure in said motor port whereby predominant motor port fluid pressure in relation to chamber pressure and valve port pressure moves said unitary valve member to open communication between said motor and valve ports for flow of fluid from said motor port to said valve port, said unitary valve member further being operative to open communication between said valve and motor ports for flow of fluid from said valve port to said motor port when the fluid pressure in said valve port exceeds that in said motor port and said chamber when the pressure in said pilot port is less than such predetermined value; said housing having a control passage between said chamber and said valve port which is opened or closed by said pilot valve member according to whether fluid pressure in said pilot port is equal to or less than such predetermined value; said pilot valve member being spring biased to a position cLosing said control passage; and said pilot valve member being operative independently of said actuating means to open said control passage responsive to fluid pressure in said motor port and chamber exceeding a predetermined maximum value overcoming such spring bias thus to relieve excess pressure for flow from said motor port to said valve port by consequent movement of said main valve member to open communication between said motor and valve ports.
 8. A load control and holding valve comprising a housing having a first passage with a main valve seat; a pilot operated relief valve in said housing comprising a spring biased main valve member movable into and out of engagement with said main valve seat and defining with said housing the movable wall of a chamber having one side thereof in restricted communication with the other side thereof; said housing having a second passage communicating said chamber with said first passage on one side of said main valve seat and having a pilot valve seat therein; a spring biased pilot valve member movable into and out of engagement said said pilot valve seat to open and close said second passage whereby when fluid under pressure on said other side of said main valve member and in said chamber attains a magnitude sufficient to move said pilot valve member out of engagement with said pilot valve seat said chamber is vented through said second passage more rapidly than fluid can be replenished into said chamber from the other side of said main valve member whereby said main valve member is moved out of engagement with said main valve seat to relieve such pressure in said first passage on the other side of said main valve seat for flow in one direction through said first passage; said main valve member being moved as a check valve member when the pressure of said one side of said main valve seat in said first passage exceeds the pressure on the other side of said main valve seat thus to permit flow of fluid through said first passage in the opposite direction; said housing having a pilot pressure passage; a piston movable in said housing to engage said pilot valve member to move it out of engagement with said pilot valve seat when the pilot pressure in said pilot passage acting on said piston rises to a value sufficient to overcome the spring bias on said pilot valve member thus to again permit movement of said main valve member out of engagement with its seat for flow of fluid in such one direction through said first passage.
 9. The valve of claim 8 wherein said piston and pilot valve member have radially overlapped surfaces which engage each other when said piston is moved responsive to such pilot pressure in said pilot passage.
 10. The valve of claim 8 wherein decrease of such pilot pressure to a predetermined value permits spring biased movement of said pilot valve member into engagement with said pilot valve seat thus to effect movement of said main valve member into engagement with its seat to stop flow of fluid in said first passage in said one direction.
 11. The valve of claim 8 wherein said first passage opens in a valve port adapted for connection with a four-way directional control valve and in a motor port adapted for connection with one motor port of a double acting fluid motor for flow of fluid into and from such motor via said first passage and said main valve seat; and wherein said pilot pressure passage is communicated with the other motor port which is connected to such directional control valve whereby when such other motor port of said motor is pressurized by operation of the directional control valve, pressure in said pilot pressure passage actuates said piston to open said pilot relief valve member as aforesaid.
 12. The valve of claim 8 wherein said housing has a stepped bore in which said pilot operated relief valve is disposed with the first passage intersecting the smallest and second smallest bore portions of said bore, with said second passage intersecting the third smallest portion of said bore, anD with said pilot passage intersecting the fourth smallest portion of said bore, said main valve seat being disposed between said smallest and second smallest bore portions, said main valve member being movable in sliding contact with the second smallest bore portion, and said pilot operated relief valve comprising a tubular body secured in said housing with a first portion extending into said second smallest bore portion and sealed thereto, with a second portion sealingly engaged with the third smallest bore portion and a third portion having screw threaded engagement with the fourth smallest bore portion, said second passage intersecting the third smallest bore portion downstream of the pilot valve seat in said body, said pilot passage intersecting said fourth smallest bore portion and acting on an annular area of said piston reciprocable in said body, said body having an axially adjustable member therein telescoped within said piston to adjust the spring biasing force against said pilot valve member, said adjustment member having a passage therethrough and said piston having a passage to communicate a chamber on the other side of said piston with said second passage to permit piston movement without trapping of fluid on such other side. 